This is invention relates to the control of a die casting machine, and in a preferred embodiment, to a method and apparatus for improving the adjustment of a velocity signal utilized to control the velocity of a moving piston in a die casting or similar machine.
The automatic feedback closed loop control of a process describes a system whereby a process variable is measured and compared to a desired value to determine the error between them, the error is used to drive an adjustment of the process to return the process to the desired value. This system has been utilized in many applications for several decades. Human observation of a process and manual adjustment of the process to achieve the desired result is satisfactory in situations characterized by two basic elements: 1. The process varies slowly as compared to the time it takes to observe, determine the appropriate corrective action and make the adjustment, and 2. Human monitoring accuracy and the resolution of the adjustment means are both sufficient to result in precision adequate to maintain the process within desired tolerances. Examples of this in common use where the person regulates the speed of the process so as to meet the above criteria, are handwriting and driving an automobile. In contrast, consider the path of a bullet resulting from shooting a firearm. Here, the person has no control over the path of the bullet once fired. The person neither can observe the path of the bullet because the process involved varies too rapidly, nor is the persons"" adjustment means during the process sufficient to maintain the accuracy of the bullet""s path on its way to the target.
The prior art has developed numerous means to enhance the means of measurement and control in order to improve over human senses and limbs, for the purpose of improving the quality and/or productivity of processes of concern. The faster the process, the more challenging are the obstacles. However, the benefits are often enormous. Modern hard disk memory densities and data storage and acquisition times have made dramatic strides in recent years which would not have been possible without constant improvements in the automatic closed loop control systems which rapidly and accurately control the position of the read/write head over a track. When the action of a closed loop control system to detect an error and make the necessary correction occurs rapidly relative to the process time constant, it is considered a real time closed loop control system. Typically, this requires the corrective action to take place in a fraction of the process time constant.
A closed loop system with a response time near to or slower than the process time constant is not real time, and has limited ability to improve process consistency and accuracy over a system without closed loop control. This is fundamentally because if the time for a closed loop system to detect and measure a deviation and then take action to return the process to the desired value is about the same or longer than the time it takes for the process to vary significantly from the desired value, then the process will be allowed to vary too far before correction takes place. The consequence will be a process with excessive variation as compared to one with real time closed loop control. Indeed, if the delay in correction is too long, the corrections can come at a time in the process when the needed correction is the opposite of the actual correction, causing the process error to increase instead of decrease as intended.
Among various processes, achieving the necessary speed of response to realize real time control also increases in difficulty as the power involved in the processes increases. High pressure metal casting (die casting) is a process which has posed difficult obstacles to the implementation of real time closed loop control because the process entails hundreds to thousands of kilowatts in the process of forming metal parts. The time to fill the die cavity and form the part is commonly in the 20 to 70 millisecond range. This is obviously too short a time for a human to measure and correct the process of filling the cavity within the filling process itself. Real time closed loop control systems have only recently been applied to controlling the velocity of the plunger used in the die casting process to inject metal into the die, with any degree of success. Early systems often resulted in the out of phase corrections described earlier and failed to improve process consistency. Often, these earlier systems responded slower than open loop systems, and thus, represented a step backward.
As a result of the foregoing, the economic benefits and quality improvements promised by real time closed loop control have fallen short of the potential or actually made things worse due to the cost of the systems and additional downtime experienced. This is due to a number of causes. The high powers and need for response times in the range of 5 milliseconds or less have required the use of high performance electro-hydraulic servo valves which exhibit variation in their transfer function as system pressure, fluid viscosity, temperature and other factors vary. Consequently, a servo valve""s output flow rates for a given input electrical signal vary not only with the above variables but also over time as they wear and/or become partially contaminated.
Prior systems for providing the electrical signal to the servo valve have used control means which measure the deviation from the desired value, and use proportional and sometimes also derivative control to generate the control signal to the servo valve. However, variations in the transfer function of the servo valve will inevitably allow a portion of the deviation to go uncorrected. For example, ideal servo valves would generate an exactly balanced flow and pressure output at each of the two output control ports in response to a zero voltage input signal, whereas real servo valves exhibit an imbalance (off-set) for a zero input. Off-sets result in velocity errors which degrade production quality and productivity. This offset is different for each valve. This alone would not pose an insurmountable problem, as such off-sets can be readily compensated for electronically. However, these off-sets vary with each of the factors listed previously. Several other variables can cause undesirable variation in the process. These include variations in temperature, hydraulic fluid viscosity, servo valve transfer function, main control valve response, hydraulic fluid supply pressure, and others.
FIG. 1 shows a block diagram of a conventional prior art system for controlling the velocity of a piston (i.e., a ram) in a die casting machine. The arrangement of FIG. 1 includes an error signal generator 101, summing circuit 102, several valves 103-104, and an operational amplifier 105. The transducer 106 may be any of a variety of commercially available off the shelf position transducers which output a voltage signal indicative of the position of, for example, a piston in a reciprocating device. The particular type of position transducer, whether it is based upon light beams, an LVDT, inductive sensors, proximity sensors, or magnetic hall effect sensors, is not critical to the present invention and many such transducers are well known and conventional in the art.
In operation, error generator 101 compares the command velocity to the actual velocity and outputs an error signal on line 108 indicative of the difference in velocity signal between the command and the actual velocity. The error signal is compared with the signal on line 109, indicative of the main valve""s position, and an output signal representing the difference there between is conveyed on line 10. A conventional amplifier 105 then causes a pilot valve 103 to change the amount of hydraulic fluid that is forced against the main valve, thereby opening or closing the main valve to change the velocity of the piston within the die casting machine. The pilot valve controls the movement of the main valve and the main valve then controls the movement of the reciprocating piston. The use of a pilot valve to control the main valve as described is known in the art and will thus not be described in great detail herein.
In addition to the problems discussed above, several problems exist with the practical application of a system such as that shown in FIG. 1. First, the pilot valve 103 usually cannot keep up with the much more rapidly varying correction signal input on line 111. Additionally, the error signal and position signal 108 and 109 would ideally bring the error down to zero. However, in the practical world, this never occurs and there always exists some error signal which is required to offset some error in the fed back position signal.
Offset error and variations in transfer function of the pilot valve, main valve and amplifier 105 all require a non-zero signal to compensate, which results in an error between actual and command velocities. Similarly, variations in RAM friction and hydraulic system fluid pressure or viscosity will result in errors in actual velocity. Moreover, variations in these factors during production require regular and repeated calibration of the systems in order to maintain acceptable accuracy.
In view of the above, it can be appreciated that there exists a need in the art for an improved technique of accurately controlling the velocity of a reciprocating piston in a die casting or similar machine. Additionally, there exists a need in the art for insuring that a pilot valve and main valve may be accurately controlled in order to permit rapid adjustment of the velocity so that a piston follows the velocity profile previously set.
For purposes of explanation, a profile is a defined set of velocities along the stroke path that the piston is intended to follow. Such profiles are calculated and stored using empirical and/or mathematical techniques known to those in the art.
The above and other problems of the prior art are overcome in accordance with the present invention which relates to a technique for compensating for virtually all of the items which hinder the ability of the control circuit to cause the velocity of the reciprocating piston to follow a specified profile. In accordance with the invention, the velocity is not controlled by a difference signal generated by feeding back the actual velocity. Instead, the velocity is controlled by the integration of such difference signal, causing the main valve of the die casting machine to continue opening or closing as widely as necessary to in order to rapidly force the reciprocating piston to follow the desired velocity profile as indicated by a velocity command signal. Additionally, the control system is supplemented by a xe2x80x9cclamp limitxe2x80x9d circuit, which overrides the control system and limits the amount of opening that the valve is allowed to experience.
While the discussion that follows is directed largely to die casting machines, the invention has applicability among a variety of reciprocating machines that need to be controlled to follow a predetermined profile.